This paper documents a series of brief, strong (Δp/p = 1), dynamic pressure oscillations that occurred in the region upstream of the Earth's bow shock during a period of radial interplanetary magnetic field (IMF). The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock‐related phenomena, recur approximately every 8–10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the Earth‐Sun line. The pressure oscillations appear to drive tailward‐moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi‐periodic variations in electron precipitation and high‐latitude ground pulsations. We use observations by spacecraft in the outer dayside magnetosphere to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.

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Most measurements of cosmic radio noise absorption in the polar regions have been made with riometers using broad‐beam (∼60 deg) antennas. Only limited information about the spatial structure and dynamics of the aurora can be obtained by this means. Recent trends in riometry have emphasized the use of multiple narrow‐beam antennas operated in a fixed beam or one‐dimensional scanning mode to examine smaller ionospheric regions. A further step in this direction has been the development of the imaging riometer for ionospheric studies (IRIS). This instrument provides a two‐dimensional image of regions of enhanced cosmic noise absorption at 38.2 MHz with a spatial resolution as small as 20 km and time resolution of 1 s. The IRIS antenna is a 64‐element dipole array, phased to produce 49 independent beams viewing an ionospheric area about 200 km square at 90 km altitude. One IRIS instrument has been operating at the South Pole station since January, 1988; a second instrument was recently installed at Sondre Stromfjord, Greenland. This paper presents a technical description of the IRIS system. The response of the IRIS to a sun‐aligned absorption arc is illustrated and compared with that of a broad‐beam riometer. An intense, localized structure that developed within the arc was observed by IRIS to propagate in the magnetic poleward direction with a speed of 1 km/s.

A series of recent studies of Pc 3 magnetic pulsations in the dayside outer magnetosphere has given new insights into the possible mechanisms of entry of ULF wave power into the magnetosphere from a bow shock related upstream source. In this paper we first review many of these new observational results by presenting a comparison of data from two 10‐hour intervals on successive days in April 1986 and then present a possible model for transmission of pulsation signals from the magnetosheath into the dayside magnetosphere. Simultaneous multi‐instrument observations at South Pole Station, located below the cusp/cleft ionosphere near local noon, magnetic field observations by the AMPTE CCE satellite in the dayside outer magnetosphere, and upstream magnetic field observations by the IMP 8 satellite show clear interplanetary magnetic field field magnitude control of dayside resonant harmonic pulsations and band‐limited very high latitude pulsations, as well as pulsation‐modulated precipitation of what appear to be magnetosheath/boundary layer electrons. We believe that this modulated precipitation may be responsible for the propagation of upstream wave power in the Pc 3 frequency band into the high‐latitude ionosphere, from whence it may be transported throughout the dayside outer magnetosphere by means of an “ionospheric transistor.” In this model, modulations in ionospheric conductivity caused by cusp/cleft precipitation cause varying ionospheric currents with frequency spectra determined by the upstream waves; these modulations will be superimposed on the Birkeland currents, which close via these ionospheric currents. Modulated region 2 Birkeland currents will in turn provide a narrow‐band source of wave energy to a wide range of dayside local times in the outer magnetosphere.

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